Yes, I was taught by my Master Sisterlocks Trainer not to Sisterlock relaxed hair; however, I have been informed by others that some Sisterlocks Trainers teach that "Sisterlocking" relaxed hair is an option. In addition, my relaxed hair was put into my Sisterlock framework and is still doing fine. So what is the best answer? Well, I have determined that it depends on your hair type as to whether or not your relaxed hair should be included into the SisterLocks framework.
Pictured above is hair from two different mannequin heads. The black strands and black Sisterlock on the left is a coarser hair that has been heat trained/processed quite a bit. The strands are more fragile and weak and very easy to tangle in the wash bowl. This type of hair CAN be put into Sisterlocks because it does not try to unravel. It may never "properly" lock, but it will be less likely to completely unravel. It will stick together. Its natural behavior is to "tangle." Even to undo the Sisterlock in this type of hair was very tedious.
Now, as for the the lighter/brown hair on the right, this type of hair is easier to unravel. I had no problem undoing the Sisterlock out of this looser patterned hair. This type of hair even in its natural state is hard to lock and hard to prevent from unraveling. So I do NOT suggest putting this type of relaxed hair into the Sisterlocks framework. Just "Sisterlock" the new growth at the roots and roller set or curl the relaxed ends in its "loose" or free state.
I hope this helps in your Sisterlocks decision making!